When using cout from multiple actors, output often appears interleaved. Moreover, using cout from multiple actors – and thus from multiple threads – in parallel should be avoided regardless, since the standard does not guarantee a thread-safe implementation.

By replacing std::cout with caf::aout, actors can achieve a concurrency-safe text output. The header caf/all.hpp also defines overloads for std::endl and std::flush for aout, but does not support the full range of ostream operations (yet). Each write operation to aout sends a message to a ‘hidden’ actor. This actor only prints lines, unless output is forced using flush. The example below illustrates printing of lines of text from multiple actors (in random order).